It was twenty years ago today that the worst nuclear accident in history took place when one of the four reactors at the Chernobyl complex 80 miles north of Kiev in Ukraine exploded. Here is the official version of events. Prior to a routine shutdown, the reactor crew prepared for a test to determine how long turbines would supply power following a loss of power, by deliberately disabling a safety mechanism that shut down the reactor automatically.

When the crew tried to shut down the reactor manually there was a dramatic power surge which caused the fuel elements to rupture. A steam explosion lifted off the cover plate of the reactor releasing radioactive material into the atmosphere. A second explosion blew out fragments of burning fuel and graphite from the reactor’s core, air rushed in and the reactor’s hot graphite core burst into flames. Fire fighters took nine days to put it out. Huge amounts of radioactive material were released…hundreds of times the fallout from the Hiroshima nuclear bomb.

Belarus took the brunt of the uranium dioxide in the plant that escaped. Ukraine was also contaminated. In total five million people were exposed to radiation including the 600 000 workers who volunteered for the clean-up operations.

The disaster occurred in the early hours of 26th April 1986. My two children were living with their mother in Uppsala 800 miles north of Chernobyl and first heard of the disaster a couple of days later when a nuclear power station in Sweden raised the alarm as a radioactive plume passed overhead and drifted north putting Sweden at the mercy of the weather. Had it rained at the wrong time my children would have been covered with radioactive fall-out.

After the disaster the city of Pripyat was emptied of inhabitants. Today it has a chilling, post-apocalyptic look to it. Ragged curtains blow through the broken windows of apartments in deserted and crumbling high-rise blocks. The streets with their Lenin statues and fading posters exhorting a march towards a communist paradise are being reclaimed by vegetation. The four dozen villages in the exclusion zone around Chernobyl were also evacuated but scores of mainly elderly people who could not adapt to the cramped city apartments they were offered have returned surreptitiously. Eventually the authorities were forced tacitly to accept their presence.

The zone’s inhabitants can collect their pensions and once every two weeks a policeman calls by to check that everyone is still alive. Adam Lahovsky, an 82-year old war veteran who lives in a small single-storey timber cottage said: ‘I was not going to allow the Chernobyl disaster to drive me out.’ His wife Nina said that a van selling bread and other staples visited once a week and they spend their pension on food and medicine.

They keep chickens and supplement their diet with berries and wild mushrooms…which have five times the permitted limit for caesium-137. Their son visits regularly to help out. Herds of boars are among the wildlife now thriving in the exclusion zone despite the radiation. Free of human predators the area provides sanctuary for moose, rare Przewalski horses and even wolves.

Evgenia Stepanova of the Ukrainian Scientific Centre for Radiation Medicine said ‘We’re overwhelmed by thyroid cancers, leukaemias and genetic mutations that are not recorded in the World Health Organisation data and which were practically unknown 20 years ago.’

In the Rivne region of Ukraine, 300 miles west of Chernobyl doctors say they are coming across an unusual rate of cancers and mutations. ‘In the 30 hospitals of our region we find that up to 30% of people who were in highly radiated areas have physical disorders including heart and blood diseases, cancers and respiratory diseases. Nearly one in three of all newborn babies have deformities, mostly internal,’ said Alexander Vewremchuk of the Special Hospital for the Radiological Protection of the Population in Vilne.

Twenty years on restrictions are still in place on reindeer in Lapland and on 274 farms in Wales and Cumbria while caesium-137 levels are ten times above permitted levels in Germany, Austria, Italy, Poland and Lithuania.